Cladding Process for Large Coke Oven Tower

Literature Overview

This 2007 study by Lin Zhusheng from Shandong Qilu Petrochemical Construction Company addresses the cladding of large coke oven towers, which are critical structures in coal coking operations. Coke oven towers are exposed to extremely harsh environments characterized by high temperatures, aggressive chemical attack from combustion gases containing SO₂, H₂S, and HCl, and mechanical wear from thermal cycling. The cladding process must provide a durable corrosion-resistant barrier while maintaining structural integrity under thermal fatigue conditions.

Core Technical Content

Coke oven towers operate in a temperature range of 200–800°C depending on the specific zone, with the upper sections experiencing the most severe corrosion. The atmospheric corrosion rate on carbon steel in the coke oven environment can exceed 1.0 mm/year, necessitating either replacement or cladding with a corrosion-resistant alloy.

Environmental Conditions and Material Selection

The corrosion mechanism in coke oven towers involves a combination of:

• Hot wet corrosion — Condensation of acidic gases on the steel surface
• Dry oxidation — At elevated temperatures above the dew point
• Thermal fatigue — Cyclic temperature changes causing cracking of the oxide layer
• Erosion-corrosion — Particulate matter in the gas stream accelerating material loss

The selection of overlay material must consider not only corrosion resistance but also thermal compatibility with the carbon steel substrate. The coefficient of thermal expansion (CTE) mismatch between austenitic stainless steel (17–18 × 10⁻⁶ /K) and carbon steel (12 × 10⁻⁶ /K) creates significant thermal stresses during heating and cooling cycles.

Process Selection and Parameters

For large coke oven tower cladding, the primary processes considered were:

1. Submerged arc welding (SAW) — For thick overlay layers on flat and slightly curved surfaces
2. Flux-cored arc welding (FCAW) — For overhead and vertical positions
3. Strip cladding — For large flat areas requiring high deposition rates

The SAW process was selected as the primary method due to its high deposition rate, good penetration, and ability to produce thick overlay layers in a single operation. The typical process parameters were:

Engineering Practice Challenges

The large scale of coke oven towers introduces several unique challenges:

Geometric complexity — Coke oven towers are not simple cylinders; they have varying diameters, conical sections, and numerous openings for gas ducts, access hatches, and instrumentation. Each geometric feature requires individual process planning and parameter adjustment.

Thermal cycling effects — The overlay layer must withstand repeated thermal cycling between ambient and operating temperatures. The CTE mismatch creates tensile stresses in the overlay layer during heating and compressive stresses during cooling. Over multiple cycles, this can lead to fatigue cracking at the overlay/base interface.

Inspection access — Large tower structures present challenges for NDT inspection, particularly for the overlay/base interface. Ultrasonic testing requires careful calibration for the austenitic overlay layer, which has a coarse grain structure that scatters ultrasonic energy.

Defect Prevention Strategy

Study Reflections

This study highlights the critical importance of material selection in high-temperature corrosion environments. The choice of 310 stainless steel for the upper tower zones reflects an understanding that the Cr₂O₃ protective film must remain intact at temperatures above 600°C, which requires a chromium content of at least 25%. The practical approach of using different overlay materials for different zones, matched to the specific corrosion mechanisms, demonstrates good engineering judgment. The emphasis on overlay thickness as a function of expected service life is particularly relevant, as the corrosion allowance must be designed to last the full inspection interval of the tower.